Testing and Buying Champions

I really wish that we could test out champions before buying them. Like if I thought Rengar looked cool and maybe would want to buy him, I would want to test him out first before making such a large purchase. The only thing I could do it wait for the next champion rotation, but I've had to wait 2 months to try out champions I thought looked cool. You can watch all the videos on a champ you want, but you won't get the feel or experience that champ.

What are your thoughts on having all champions available in practice mode or something so that we can try out any champion that we wanted before purchasing them?